In process equipment used for the fabrication of semiconductor devices, accurate control of process temperature is important for well-controlled reactions and/or film growth to be accomplished. Representative of a kind of the process equipment in which temperature is the most important setting condition are the so-called furnace bodies of the oxidation furnace or the like. The furnace bodies of this kind are filled with an oxidizing atmosphere in place of the atmospheric air. The substitution atmosphere in this case is at atmospheric pressure or higher. In the furnace, a silicon wafer, for example, is heated by radiation of a heater installed around a quartz tube and also by heat transfer by the atmosphere at ambient pressure in the quartz tube. To be more specific, since there is a heat transfer medium, the temperature can be measured relatively accurately with a measuring element such as a thermocouple located in the heat transfer atmosphere.
To give an example in which a heat transfer medium is not used, still, in which the temperature is well controlled, it is the baking apparatus for photoresist used in the process step of applying a photoresist as the mask in the etching process. In this baking apparatus, photoresist is baked in the atmosphere at atmospheric temperature. The silicon wafer is placed on a heat block with a heat capacity greater than that of the silicon wafer heated to a specified baking temperature. Then, the silicon wafer is pressed with its whole surface against the heat block side by the atmospheric pressure by the use of a vacuum chuck. The temperature of the wafer comes to be in equilibrium with the temperature of the heat block, so that the wafer temperature can be controlled accurately by a temperature measuring element such as a thermocouple mounted on the heat block.
Most of the semiconductor device manufacturing processes utilize well-controlled reactions in a dust-free environment, and therefore, often require processing in a vacuum. Hitherto, it has been fundamentally difficult to implement precise temperature control of the wafer in a vacuum in the manufacturing of semiconductor devices. The reasons are as follows.
In heating the wafer with a lamp heater, due to the absence of a heat transfer medium, the wafer is heated only by radiation. Therefore, as is well known, a very small amount of heat is absorbed by the mirror-like surface of a metal due to its high reflectance, while a large amount of heat is absorbed by a black body. Accordingly, the degree of heat absorption differs greatly with the surface condition of a wafer to be heated.
By using a thermocouple attached to a wafer, an attempt has been made to accurately measure the temperature of the wafer during processing. In this case, however, the wafer temperature is measured with a thermocouple in point contact with the wafer, and this contact condition of the thermocouple is difficult to maintain stably. The result is a poor reproducibility of the measured temperature.
When a silicon wafer is heated by infrared radiation, due to the fact that silicon wafer is substantially transparent in a wide area of the infrared region, not only the heat travels to the thermocouple by heat transfer from the wafer but the thermocouple is directly heated by the lamp heater. This makes it difficult to accurately measure the wafer temperature.
Also, there is a method of forcibly bringing a heat transfer medium into a vacuum. For example, as disclosed in JP-A-56-48132 or JP-A-58-213434, a silicon wafer is clamped to a heat block located in a vacuum, a gas is filled between the backside of the silicon wafer and the heat block with a pressure of about 1 Torr, thereby making the wafer temperature to be in equilibrium with the heat block temperature. Also in this case, the equilibrium wafer temperature to the heat block is measured by a temperature measuring element such as a thermocouple mounted to the heat block.
In this example, however, the uniformity and reproducibility of temperature may not be sufficient because the wafer is clamped to the heat block by a small force compared with a case where a vacuum chuck is used under atmospheric pressure. The greatest disadvantage of this heating method is that due to the low density of the heat transfer medium, it takes time for the heat be conducted from the heat block to the wafer. Even if a thermal equilibrium is reached the heat block and the wafer eventually, as described with reference to the above example, it takes several to several tens of second, and various factors are considered to affect the reproducibility of the heat transfer time. If the damper fails to clamp the wafer properly, the wafer temperature dose not reach the equilibrium, and, hence, accuracte temperature can be known by no means.
As has been described, whichever heating means is used, it is necessary to measure the wafer temperature with no contact with the wafer in a vacuum. As one method of this type, a method has been proposed which measures the radiation intensity from the wafer in the infrared region by using an infrared thermometer.
Specifically, this method is to mount a wafer on the heat stage in a sputter-deposition apparatus and, while heating, to measure the wafer temperature with an infrared thermometer through a hole in a target placed against the wafer. To be more concrete, the infrared emissivity of the wafer at specific temperatures is measured in advance using a calibration specimen, and according to the measured values, the wafer temperature is controlled during sputtering.
An example related to this technique is revealed in JP-A-1-129966.
In this method, however, the emissivity of the wafer is not necessarily constant as described below, so that it is difficult to implement an accurate temperature measurement and there are some problems.
To be more specific, as a specimen for calibration, a silicon wafer is used to which is deposited with several hundreds of .ANG. of aluminum, for example. The infrared emissivity observed by an infrared thermometer from the wafer surface differs with the presence or absence of a metal on the front side of the wafer, making it impossible to control temperature without knowing the emissivities before and after film deposition.
After film deposition is started, accurate temperature measurement cannot be performed until a film has been deposited to a certain degree of thickness (500 to 1000 .ANG. of aluminum, for example).
When a metal film is desposited, a mirror-like surface is formed, which reduces the emissivity to a very small value, thus making measurement very difficult.
There is a problem which makes it difficult to perform an accurate wafer temperature measurement in a vacuum and temperature control that goes with it. The problem is that the infrared emissivity differs with different lots of wafers. If the method is used in which another wafer for calibration is prepared as in the above example, accurate temperature control of wafers cannot be implemented because the wafer for calibration is different from wafers on which a film is actually deposited, accurate wafer temperature control cannot be performed.
In the vacuum processing apparatuses which have been heretofore used, various temperature control means have been used, but there has been no apparatus capable of controlling the wafer temperature by accurately knowing the in-process temperature.
An ideal method of wafer temperature control using a infrared thermometer is to calibrate the infrared thermometer using a wafer on which a film is actually deposited and to thereby make it possible to measure the wafer temperature regardless of the presence or absence of a deposited film and the difference in the infrared emissivity resulting from the film condition. However, no method applicable to practical use has been proposed.